1. Field of the Invention
The invention relates to an electrodynamic device comprising a movable permanent-magnetic member (rotor) and an electromagnetic stator comprising a substrate such as a sheet or foil, a plurality of windings comprising spiral first and second coils which are electrically connected in series with one another via interconnections and which are situated, respectively, on the first and the opposite second side of the substrate, and a plurality of phases each comprising at least one winding.
As is common usage, "rotor" is to be understood to mean the moving member of the device. This implies that it need not be a rotating member although it may be such a member. Apart from being rotating the rotor may be translating or may perform a combined translation and rotation. The rotor may be disc-shaped or cylindrical. If the device forms an electric motor it may be of the axial-field type or the radial-field type.
2. Description of the Related Art
It is known (WO 90/07220) to provide a device of the type defined in the opening paragraph with a stator comprising a plurality of stacked substrates, each substrate carrying a plurality of series-connected coils together forming one of the phases of the device. Said device forms an electronically commutated axial-field motor of flat construction. Motors of this type are used increasingly, for example, for driving information discs which can be read without mechanical contact, such as optical discs or magnetic discs of magnetic disc storage units with aerodynamically floating magnetic heads (so-called Winchester drives).
In the known device the number of stacked substrates is always equal to or a multiple of the number of phases. To avoid loss of space the coils on each substrate are arranged on the substrates in a regular pattern which is as compact as possible. This pattern should correspond to the pattern of the magnetic poles of the rotor. This is because all the coils belonging to one phase are always energized at the same time. Thus, with the known device it is not possible to have a number of coils per phase differing from the number of rotor poles without loss of space. However, in many cases such a different number is desirable in order to minimize pulsations in the torque exerted on the rotor and, consequently, variations in the speed of movement of the rotor. This is of particular importance in the case of small motors having a low-mass rotor in order to guarantee the desired stable operation of the device.
Particularly in the case of a rotor of small dimensions the magnitude of the magnetic-field component contributing to the Loreritz forces depends strongly on the distance from the rotor. In the known device the driving force exerted on the rotor will therefore differ per phase, depending on the distance of the relevant substrate from the rotor, for a phase current of the same magnitude per phase. This has also an adverse effect on the stability of operation.